We present the results of an investigation of the one channel Hp photometry
made by the Hipparcos satellite and presented in the Hipparcos and Tycho
Catalogues (ESA, 1997) for 24 variable super-and hypergiants: the
Cyg variables. The observations were obtained between November 1989
and March 1993 and are presented in Fig. 1 (click here) to
Fig. 3 (click here). These massive stars show without
exception photometric micro variations, also referred to as
Cyg-type variations, provided that they are evolved and massive enough
(
). The OB-type stars in this group show
brightness variations usually 
, the later type stars may show
brightness variations of the same order, but often also larger variations. For
the early type stars the typical time scales for the variations is 2-6 weeks,
but for later type stars it can reach a few hundred days
(van Genderen 1989, 1991).
Due to their non-strict periodicity, i.e. light curve shapes vary from cycle-to-cycle, the "periods'' are in fact "quasi-'' or "pseudo-periods''. It is quite well possible that the cause for this must be sought, partly or largely, in a multi-periodic origin (van Genderen et al. 1989; Sterken et al. 1997) together with possible effects of additional large scale atmospheric motion fields (e.g. de Jager et al. 1984; Burki 1987; van Genderen 1991) and other stochastic variations (Sterken et al. 1997).
Figure 1:
Light curves for the first 8 stars as Hp versus Baricentric Julian Day
Figure 2:
Light curves for the second group of 8 stars as Hp versus Baricentric Julian
Day
Figure 3:
Light curves for the last group of 8 stars as Hp versus Baricentric Julian Day
The variables presented here were part of a larger sample, and were selected on account of their reputation and/or their relatively large light variations. This selection contains 3 SMC-, 12 LMC- and 9 Galactic members, of which 4 S Dor-type variables or LBVs and 2 possible members of this class. For each of these stars a period analysis was performed, the results of which were compared with previous studies if available.
A comparison has been made between the Hp magnitudes and magnitudes in the
Johnson V band, 
. These were either obtained with a UBV photometer,
or based on the transformation to of the Walraven V or Strömgren y
band. Observations in the latter system have been obtained by the LTPV project
(Long-Term Photometry of Variables) led by Sterken (1983).